Using vacuum to increase the insulating performance of window glazing components is not a new concept, and in fact many innovative approaches have been taught in the literature over the last 75 years. It is, however, readily observed by skilled practitioners of the art that the majority of the prior work relates to low- to medium-vacuum levels, i.e., vacuum levels within the range from about 760 torr to about 10−3 torr. Note, for purposes of this application, a “higher” level of vacuum is understood to correspond to a lower absolute pressure, e.g., a vacuum level of 10−4 torr is a higher vacuum than 10−3 torr. In a few cases the literature makes reference to the measured vacuum levels in glazing components, but in many cases the maintainable vacuum level must be interpreted from careful evaluation of the materials exposed to the vacuum enclosure, the methods used to create the vacuum seal and the methods used to produce the vacuum condition in the enclosed space.
While the literature describing vacuum insulating window glazing components may not rigorously define the vacuum levels, literature from other industries, such as the electronics industry, defines different vacuum levels and the types of materials and processing methods required to achieved and maintain those specified vacuum levels. The common distinction between medium- and high-vacuum devices is a vacuum level of 10−3 torr. In other words, the range of high-vacuum levels begins at about 10−3 torr and goes higher, i.e., in the direction toward and/or past 10−4 torr. In the case of vacuum insulating window glazing components, where it is desirable for the components to retain a prescribed minimum vacuum level for an extended operating lifetime (e.g., 25 years), a vacuum containment system capable of initially maintaining a higher level of vacuum (e.g., 10−5 torr), may be necessary. For the purposes of this application, vacuum insulating glazing units capable of maintaining vacuum levels of 10−3 torr or higher are termed high-vacuum insulating glazing units (HVIGU).
One purpose of HVIGUs is to provide lower levels (i.e., compared to units with low- or medium-vacuum levels) of conductive heat losses between temperature-controlled spaces and non-temperature-controlled spaces separated by the glazing unit. In such cases providing this desired lower level of conductive heat loss over a long period of time is desirable. Since the ambient conditions in the uncontrolled space, most commonly the external atmospheric environment, produce a variety of stresses, including thermal, pressure and mechanical vibration and since, to a lesser extent, this also happens also in the conditioned space, various embodiments of the HVIGU will be more or less capable of surviving the applied stresses while maintaining the desired minimum vacuum level. Thus, the design lifetime, i.e., the period of time that the HVIGU will maintain its level of performance, is one of the performance features of the HVIGU.
Generally speaking, HVIGUs are typically constructed using at least two spaced-apart panes of glass of some prescribed thickness. These panes are then sealed, typically along the edges, using some arrangement of sealing elements which are intended to isolate the evacuated volume from the surrounding atmospheric pressure. Since the primary objective of the HVIGU is to provide a low thermally-conductive barrier between environmental spaces, each of which may have a higher or lower temperature with respect to the other, it is obvious to skilled practitioners of the art that the two panes of glass may reach temperature levels which vary distinctly from each other. In fact, for a given space-to-space temperature differential, the pane-to-pane temperature differential will typically increase as a function of reduced thermal conductivity of the HVIGU. As a result of the temperature differential between the panes of glass, the panes may expand and contract differentially. This may introduce substantial strain at the edges of the HVIGU where the seal is attached. If the seal at the HVIGU edge is made to be rigid, pane-to-pane temperature differentials may produce significant stresses in the HVIGU, along with a number of expected deleterious effects, for example, large-scale deflections, bowing and other physical or optical changes of panes and/or shortened seal life for the HVIGU.
A need exists, therefore, for a flexible edge seal for a HVIGU or other insulated glazing unit that can accommodate the strains associated with the expanding and contracting glass panes. A need further exists, for a flexible edge seal that can withstand the mechanical forces imposed by atmospheric pressure on the seal. A need still further exists, for a flexible edge seal that can retain the prescribed vacuum levels within the evacuated space.